1. Field of the Invention
The present invention relates to radio communication systems and, more specifically, to a method for setting up a stable call in a radio communication system using dynamic channel allocation.
2. Description of the Related Art
Conventionally, as depicted in FIG. 3A, upon cell set-up between a radio terminal and a radio communication system, a base station performs the call set-up while checking whether or not a call request exists from a subscriber terminal. Initially, the base station checks all available traffic channels under service for a traffic channel having more than a predetermined RSSI (received signal strength indicator) level. In the radio communication system having the above dynamic channel allocation, the traffic channel is dynamically allocated in the base station and the subscriber terminal when an outgoing call request from the subscriber radio terminals 100a, 100b, and 100c is made, as shown in FIG. 1. Namely, when the radio subscriber requests the outgoing call at his position through the radio terminal 100a, the radio terminal 100a selects the channel having little interference and transmits the call request message to the base station 110a. The call set-up in the radio communication system is achieved by exchanging the channel information of the base station and the subscriber radio terminal by using only the corresponding traffic channel between the base station and the subscriber radio terminal in determining the radio traffic channel used by the new call set-up. Therefore, since the radio communication system using the fixed channel allocation re-uses the radio channels, it is understood that the limited radio channels provide service to a large amount of traffic. However, the allocations of channels requires careful engineering analyses of radio propagation conditions and traffic patterns within and between the cells. Fixed allocation cellular radio systems are engineered so that harmful levels of interference from signals used in neighboring cells are unlikely to be encountered.
Micro-cellular radio systems tend to use dynamic, rather than fixed, channel allocation. With dynamic channel allocation (DCA), each cell can potentially use any traffic channel for a given call of those allocated to the entire service. The decision as to which channel to use is made dynamically, i.e., based on channel usage conditions experienced at the time of call set-up. The decision is also made in a decentralized manner, i.e., by the base station or subscriber terminal in question, without any centralized coordination with other cells. This has an advantage in that no detailed engineering analyses are required for each cell site, cell sites can be easily added or moved, and more traffic can be handled than with fixed allocation because unused channels in neighboring cells can be borrowed.
In present systems incorporating DCA, the determination as to which channel is to be used for a call set-up is made according to channel information from either the base station or the subscriber terminal. The disadvantage with this approach is that appreciable interference may be present on that channel from the viewpoint of the other end of the link. For example, a traffic channel may have little appreciable interference, as monitored by the base station. However, the same traffic channel may have unacceptable interference at the subscriber terminal.